For the second time in less than a week, a north Dallas home caught fire last night, giving investigators plenty of reason to think the blaze was no accident.  Just before 11 p.m. Tuesday, Dallas fire crews rolled up to a house on Melshire Drive, just east of the Dallas North Tollway near LBJ Freeway.  They immediately realized it was the same house where they doused a fire Friday night.  No one has been living in the home since then, so fire investigators now want to know why and how a fire erupted in a vacant home.